1. Field of the Invention
The present invention relates to an optical scanning device, an optical scanning method, and an image forming apparatus.
2. Description of the Related Art
An optical scanning device and an optical scanning method are widely used in existing image forming apparatuses such as digital copiers, printers, and digital multi function peripherals (MFP). A typical optical scanning device and an optical scanning method cause a light deflector to deflect a laser beam from a laser light source, form a beam spot on a scanning surface, i.e., a surface to be scanned, by using a scanning-image-forming optical system, and carry out optical scanning.
An edge-emitting semiconductor laser (edge-emitting laser light source) has been predominantly used as a laser light source in the existing optical scanning devices. However, in recent years, a surface-emitting laser light source (or a vertical cavity surface emitting laser (VCSEL)) is used in place of the edge-emitting semiconductor laser. As compared to the edge-emitting laser light source, array arrangement of light-emitting portions is easy in the surface-emitting laser light source. In the existing edge-emitting semiconductor laser arrays, an array sequence of four to eight light-emitting elements can only be arranged. However, if the surface-emitting laser light source is used, an array sequence of 16 to 32 or even more light emitting portions can be arranged. In other words, the surface-emitting laser light source is highly anticipated as a light source of a multi-beam optical scanning device.
A diffractive optical element is generally used in optical scanning devices. For example, in the existing optical scanning devices, laser beam that is deflected by the light deflector is detected outside of an effective scanning area on an optical scanning start side and the result of the detection is used for synchronization control for starting optical writing. Moreover, a technique has been proposed in which the laser beam is separated into two light beams of a zero-order light and a first-order diffracted light by using the diffractive optical element, one of the light beams is detected and the result of the detection is used to perform the synchronization control for starting optical writing while the other is used to detect a scan line position in a sub-scanning direction.
Thus, a diffractive optical element is used as an optical element in the existing optical scanning device. For example, an optical scanning device that includes the surface-emitting laser light source and the diffractive optical element is disclosed in Japanese Patent Application Laid-open Nos. 2007-133385 and 2007-199666.
The optical scanning device includes members or units such as a polygon mirror, which is used as the light deflector, and a semiconductor laser light source that produce heat when in use. Although the temperature (inside temperature) of the optical scanning device varies according to the season, the temperature generally fluctuates within a range of 30 degrees. It is also well-known that a luminescence wavelength of the laser light source, which is used as the light source, also varies according to temperature variation. The same phenomenon can also occur in the surface-emitting laser light source.
Various optical elements that are used in the optical scanning device are designed such that those optical elements perform desired optical functions at a standard temperature. This standard temperature is set for each optical scanning device according to the design of the optical scanning device and it is generally around 25° C.
A diffraction angle, by which the diffractive optical element diffracts an incident light, changes when a grating pitch of a diffraction grating changes. Furthermore, the diffraction angle also changes when a wavelength of a diffracted light changes.
If the diffractive optical element is made of plastic material, the grating pitch of the diffraction grating changes due to thermal deformation of the diffractive optical element that occurs due to fluctuation in the temperature of the optical scanning device. Due to this, the diffraction angle of the diffracted light also changes and is likely to stray from a light detector that detects the diffracted light. One approach to this problem could be, as disclosed in Japanese Patent Application Laid-open No. 2007-199666, to make the diffractive optical element with an optical glass having extremely insignificant thermal deformation. However, even if the diffraction grating having insignificant heat deformation is used, changes in the diffraction angle that occur due to variation in the luminescence wavelength of the light source are not avoided.
In the above-mentioned conventional technologies, measures have not been taken with respect to changes in the diffraction angle that occur due to thermal deformation of the diffraction grating itself and changes in the diffraction angle that occur due to changes in the waveform.